4.1 (8) - DNA, Genes and Protein Synthesis

Question 1

What 3 things is a nucleotide made from?

Answer 1

• Phosphate group
• Deoxyribose sugar (pentose)
• Nitrogenous, organic base

Question 2

What are the base pairing rules?

Answer 2

A + T (held together by 2 hydrogen bonds)

G + C (held together by 3 hydrogen bonds)

Question 3

What is DNA?

Answer 3

DNA is a hereditary, information-carrying molecule

Question 4

What is the minimum number of bases needed to code for an amino acid? Why?

Answer 4

3, because pairs wouldn’t be able to produce the range of amino acids we need

Question 5

What are some features of the genetic code?

Answer 5

• Degenerate code
• Triplets always read in the same way
• Have start and stop codes in DNA sequencing
• Each sequence is only read once
• Universal code

Question 6

What is a degenerate code?

Answer 6

When most amino acids are coded for by more than one triplet (eg. an amino acid could be made from ATT or AAG).

Question 7

What is a universal code?

Answer 7

When each triplet codes for the same amino acid in all organisms

Question 8

Which direction does the left strand run in?

Answer 8

5’ to 3’

Question 9

Which direction does the right strand run in?

Answer 9

3’ to 5’

Question 10

What are some functions of antiparallel strands?

Answer 10

• Maintains 2 copies of the DNA, in case of mutation
• Protects bases from reacting with other bases
• Required for semi-conservative replication
• Improves molecular stability

Question 11

What is a gene?

Answer 11

• A unit of heredity

- A section of DNA on homologous chromosomes coding for one or more polypeptides

Question 12

What is DNA like in a eukaryotic cell?

Answer 12

• DNA in a double membrane-bound nucleus
• Wound round histone proteins, to condense it
• Lots of linear chromosomes
• No plasmids
• Has a lot of introns (DNA that don’t code for polypeptides)
• Has exons (coding sequences within genes)
• Some genes code for rRNA and tRNA

Question 13

What is DNA like in a prokaryotic cell?

Answer 13

• DNA is free in the cytoplasm
• Naked
• Single circular chromosome
• May contain plasmids (smaller loops of DNA)

Question 14

What is a homologous chromosome?

Answer 14

2 chromosomes (1 maternal and 1 paternal) that have the same loci, which allows them to determine the same features

Question 15

What is a human’s diploid number?

Answer 15

46 (23 homologous chromosomes)

Question 16

What is a locus?

Answer 16

The specific point on a chromosome where a gene is located.

Question 17

What is a genome?

Answer 17

The complete set of genes in a cell

Question 18

What is a proteome?

Answer 18

The full range of proteins produced by a genome

Question 19

Give 4 ways in which RNA is different to DNA

Answer 19

• RNA is made with ribose nucleotides, DNA is made with deoxyribose nucleotides
• RNA has the base ‘uracil’ instead of ‘thymine’
• RNA is single-stranded, DNA is double-stranded
• RNA is shorted than DNA

Question 20

What is the molecular difference between deoxyribose and ribose?

Answer 20

On the 2’ carbon, ribose has a hydroxyl group

Deoxyribose doesn’t have this

Question 21

What are the 3 types of RNA?

Answer 21

mRNA, tRNA and rRNA

Question 22

What is the function of RNA?

Answer 22

It transfers coded information from DNA into the cytoplasm where is can be translated into proteins.

Question 23

What is the function of mRNA?

Answer 23

• The code for a gene in DNA is transcribed (copied) into an mRNA molecule.
• mRNA carries the code for a particular polypeptide from the nucleus to the cytoplasm. It can travel through nuclear pores.

Question 24

What is the function of tRNA?

Answer 24

tRNA matches the codons in an mRNA molecule to their constituent amino acids

Question 25

Describe a tRNA molecule

Answer 25

• On the middle loop, there is a triplet nucleotide sequence called an anticodon, which is complementary to one of the 64 possible codons on the mRNA.
• Each amino acid is attached to a tRNA with the appropriate anticodon.

Question 26

What is the function of rRNA?

Answer 26

Together with proteins, it forms ribosomes

Question 27

What are codons?

Answer 27

Triplets of base pairs on a mRNA molecule.

Question 28

What is transcription?

Answer 28

The process of making pre-mRNA using part of the DNA as a template

Question 29

If a coding DNA sequence is CGA, what is the pre-mRNA sequence?

Answer 29

GCU

Question 30

If a pre-mRNA sequence is UCA, what is the coding DNA sequence?

Answer 30

AGU

Question 31

What is a DNA template (coding) strand?

Answer 31

A DNA template strand is the strand used by DNA polymerase to attach complementary bases to during DNA replication

Question 32

Describe the 6 stages of transcription

Answer 32

1) DNA helicase breaks the hydrogen bonds on the DNA molecule, exposing its bases.
2) RNA polymerase binds to a section of DNA.
3) The nucleotides on 1 of the 2 DNA strands (template strands) bind to complementary mRNA nucleotides present in the nuclei.
4) RNA polymerase moves along the strand, joining the nucleotides together to make a pre-mRNA molecule.
5) While mRNA nucleotides are attaching, the DNA strands re-join behind the newly-forming strand of pre-mRNA.
6) When the RNA polymerase reaches the stop codon (end of the triplet code), it detached and the pre-mRNA is complete.

Question 33

Explain and describe the process of splicing pre-mRNA

Answer 33

1) The non-functioning introns are later removed from the pre-mRNA sequence.
2) The remaining exons are joined together by splicing to form mature mRNA.
3) The exon sections can be joined together in a variety of combinations and orders, so the same sequence of DNA can code for several different proteins

Question 34

Why doesn’t the splicing of pre-mRNA happen in prokaryotes?

Answer 34

They don’t have introns, so it isn’t necessary.

Question 35

What is translation?

Answer 35

Translation is the process of converting the codon sequence in the mRNA molecule into an amino acid sequence

Question 36

Describe the 7 stages of translation

Answer 36

1) mRNA molecule leaves the nucleus via nuclear pores after splicing
2) A ribosome attaches to the start codon of the mRNA
3) A tRNA molecule, with the complementary anticodon sequence to the first mRNA codon, pairs with the codon on the mRNA
4) The ribosome has 2 codon binding sites, so 2 tRNA molecules at a time are brought together
5) A peptide bond forms between the 2 amino acids on the tRNA with the use of an enzyme and ATP (ATP hydrolysed into ADP and Pi to provide energy)
6) The ribosome moves along the mRNA to the next codon, so the first tRNA is released and a new tRNA arrives at the next codon
7) This continues, with up to 15 amino acids being added every 15 seconds, until a polypeptide chain is built